Computer-Generated Pen-and-Ink Illustration

1. Computer-Generated Pen-and-Ink Illustration Georges Winkenbach and David H. Salesin University of Washington SIGGRAPH 1994

2. NPR illustrations Pen-and-Ink: some advantages over realism • Convey information better • Consume less storage • Easily reproduced and transmitted • Vary the levels of detail • Add sense of artistic vitality

3. Pen-and-ink illustration • Strokes • Tones and Textures • Outlines

4. Strokes Strokes are produced by placing the ‘nib’ of a pen in contact with the paper and tracing a path • Too thin a stroke can give a washed-out appearance • Too coarse a stroke can detract from delicate details • Pen position is often altered by turning the nib during a stroke to create artistic inconsistencies • Even-weighted line drawings appear lifeless. The thickness of a line should vary along its length • Wavy lines indicate hand motion and make images appear less computer rendered

5. Tone/Textures Tone is a function of the ratio of black ink to white paper over a certain region of the illustration Texture is created when the character of the strokes is varied over the same region • In traditional pen-and-ink illustration, combinations of strokes are used to create an overall impression of the desired tone • Pen-and-ink drawing is unique in that combinations of strokes create both the tone and the texture, and these elements cannot be separated

6. Tone/Texture • Tones should be created from lines of roughly equal weight and spacing • To clarify distinctions between similarly toned objects, it is sometimes important to enhance contrast or invent shadows • Strokes are important for conveying texture, as well as depicting geometry and indicating lighting

7. Tone/Texture • ‘indication’ is used to imply a texture without drawing every single stroke. It provides a means to emphasize selected portions of the image

8. Outlines Outlines depict the borders of an image and distinguish between the shapes within that image • The quality of the outline stroke is important for conveying texture • Thick outlines are used to suggest shadows or bring objects forward in the scene • Outlines should become ‘haloed’ and fade away where one object passes behind another • When tones are not used, outlines are required to convey the shape • Using ‘indication’ when drawing outlines is just as important as when drawing tones

9. Outlines

10. graphics rendering pipeline • Modifications • Standard aspects • Differences

11. Modifications • dual nature of strokes • traditionally, texture and tone are rendered independently. However, in pen-and-ink, the same strokes that produce tone also convey texture • need to combine 2D and 3D information • traditionally, rendering info is entirely 3D with final projection to 2D largely a matter of sampling the rendered shades • For pen-and-ink, size of projected areas must be used to compute proper stroke density in order to accommodate dual nature of strokes (tone/texture) • Also, outlining depends on type of junction between 2D boundaries and level of contrast between tones of adjacent 2D regions

12. Standard aspects of pipeline • the model (a standard 3D polygonal model) • assignment of texture • lighting model • visible surface algorithm • shadow algorithm

13. Differences • maintaining 2D spatial subdivision • rendering of texture and tone- polygons are no longer scan converted. Texture and tone must both be conveyed through hatching • clipping- stroke based clipping to texture region maintains hand-drawn effect • outlining- ‘boundary outlines’ surround visible regions, ‘interior outlines’ used within polygons, suggest shadow direction or give view-dependent accents to stroke texture

14. Strokes • Waviness function • Pressure function

15. Strokes

16. Textures Prioritized stroke texture • When rendering a prioritized stroke texture, highest priority strokes are drawn first; if rendered tone is too light, next highest priority strokes are added, etc. until you get proper tone

17. Textures

18. Textures Resolution dependence • Problem: enlargement is performed either by pixel replication, which yields ugly aliasing artifacts, or by drawing at higher resolution which yields thinner strokes and therefore lighter illustration. Reduction is performed by scan-converting curves at lower resolution, yielding a ‘large black mass of overlapping strokes’ • Solution: Prioritized stroke texture chooses proper texture and tone for given size and resolution

19. Textures Indication • It is important to suggest texture without drawing every last stroke • They implemented a semi-automatic method where user specifies where detail should appear, and indication is used everywhere else

20. Textures

21. Textures Indication • A field w(x,y) is generated by detail segment l at point (x,y) in texture space according to: w(x,y)= (a + b * distance((x,y), l))^-c where a,b and c are non-negative constants that can change the effect of the field • The field w(x,y) is perturbed by a small random value so as not to create patterns that are too regular

22. Outline Boundary outlines • Surround visible polygons of the image • Take into account both textures of surrounding regions and the adjacency info stored in the planar map • Each stroke texture has a ‘boundary outline texture’

23. Outline Interior outlines • used within polygons to suggest shadow directions or give view-dependent accents to stroke texture

24. Outline Minimizing outline • Let E be an edge that is shaded by two faces F and G of a planar subdivision. E is drawn only if the tones of face F and G are not sufficiently different for the two faces to be easily disambiguated by their shading alone

25. Outline

26. Outline Accenting outline • Brick edges that cast shadows are rendered with thickened edges, while illuminated brick edges are not drawn at all

27. Outline Viewing direction alterations • Viewed from above, all edges between individual shingles are clearly visible; viewed from the side, the shingles tend to blend together and vertical edges begin to disappear • Each stroke texture is outfitted with a simplified Anisotropic Bi-directional Reflectance Distribution Function which describes its outline features in terms of both lighting and viewing directions

28. Outline

29. Contributions of this paper • Surveyed established principles from traditional illustration that can be used for communicating visual information effectively • Showed that a large number of these principles can be incorporated as part of an automated rendering system, and that the information present for driving the ordinary graphics pipeline is in many respects also sufficient for achieving important non-photorealistic effects

30. Contributions of this paper • Introduced the concept of a ‘prioritized stroke texture,’ a general framework for creating textures from strokes, and provided a methodology for building procedural versions of these textures • Allowed a form of resolution-dependent rendering, in which the choice of strokes used in an illustration is appropriately tied to the resolution of the target medium